1. Field of the Invention
This invention relates to zoom lenses, and, more particularly, to zoom lenses suited for a range including long focal lengths.
2. Description of the Prior Art
In the past, as one type of zoom lens there has been well known the objective lens system comprising, from front to rear, a varifocal section consisting of three lens groups having positive, negative and positive refractive powers distributed in this order and a fixed section. The 2nd lens group is axially moved to effect a change in the focal length. The 3rd lens group is axially moved in a reciprocating manner to compensate for the image shift due to the zooming. In this type of zoom objective lens, the 1st lens group for focusing remains stationary during zooming. This gives the advantage that the distance from the front vertex of the lens to the image plane, or the total length of the lens does not change between the wide angle and telephoto settings. However, a disadvantage of this type zoom objective is that an increase in the zoom ratio and relative aperture generally calls for rapid increases in the total length of the lens and the diameter of the front lens members, thereby contributing to an extreme loss in compactness. For this reason, most of the known zoom objectives of the type described, as, for example, disclosed in Japanese Patent Sho No. 49-899, Japanese Laid Open Patent Nos. Sho 51-37247 and 51-63635, and U.S. Pat. Nos. 4,223,981 and 4,240,699 have the ratio of the total length of the lens in the telephoto side to the focal length, that is, the telephoto ratio at about unity and even in the shortest case, on the order of 0.97. Accordingly, since determination of the total length of the lens is made by taking into account the possibility of achieving good correction of aberrations at the telephoto settings, because the total length of the lens remain unchanged during zooming, The result is that, at the wide angle settings, the lens is considerably bulky as compared with fixed focal length lenses having the equivalent focal length. Thus, this constitutes a large burden when seeking a place where the lens with the camera body is to be stored, or when the lens is being carried about.
Attempts have been made, therefore, to eliminate the above-described drawbacks and at the same time to achieve a high grade imagery with a reduced number of lens constituents by choosing such zoom type that the total length of the lens expands in the telephoto positions and by having a good compromise between the requirements of giving an appropriate power distribution and employing cam slots of moderate curvature controlling the movement of the lens groups. As examples of zoom objectives of the character described, mention may be made of U.S. Pat. No. 3,784,283 and Japanese Laid Open Patent No. Sho 55-161207, where the varifocal section is formed with a 1st lens group of positive power and a 2nd lens group of negative power, and where the 2nd lens group is moved axially in one direction during the entire zooming range to vary the focal length, while simultaneously the 1st lens group being axially moved reciprocatingly to effect image shift compensation. Since, in this method, however, the 1st lens group which has a relatively large outer diameter and a heavy weight is moved along a tortuous camming slot, a strong stress is laid on the operating mechanism in the lens mounting, and the tolerances within which the parts of the mechanism may be manufactured economically are unavoidably deteriorated badly. Thus, it has found its use mainly in lenses for 8 mm film.